Device for protecting an electrical circuit against interference pulses

ABSTRACT

A protection device for suppressing higher frequency interference pulses in an input signal for an electric circuit includes a transistor whose emitter is grounded via a current source to act as an emitter follower for an input signal. The voltage drop of the base-emitter diode of the transistor is compensated for by a corresponding voltage drop across a diode whose anode is connected to a voltage supply line via a second current source. A junction point of the second current source and the diode is connected to a capacitor which together with the second current source forms a low-pass member for the input signal. A Schmitt-trigger circuit is connected to the junction point to restore lower frequency input pulses from trapezoidal pulses picked up at the capacitor.

BACKGROUND OF THE INVENTION

The present invention relates to a device for protecting input signalsof an electrical circuit against interference pulses of a higherfrequency.

Such a device is also known from the DE-PS No. 28 32 766. DE-OS No.2832766 discloses a protection device comprising a capacitor whichsubstantially determines the time constant of a low-pass filter whichacts against higher-frequency interference signals or interferencepulses. The low-pass filter substantially comprises a transistor whichis switched via the capacitor into its conductive state when aninterference pulse occurs and accordingly loads the source ofinterference at a low impedance. Low-frequency signals do not lead to anactuating of the transistor, so the overall arrangement acts like alow-pass filter.

A substantial disadvantage of this known protection device consists inthat a very large capacitor is required, which does not readily permit amonolithic integration of the protection device, because the capacitorswhich can be realized in integrated circuits at a reasonable expense arein the order of magnitude of 100 pF. Moreover, currents can only berealized reasonably down to the order of magnitude of 0.5 uA. For evensmaller currents, the limit is set by blocking currents, which sharplyincrease at high temperatures in particular. In practice, therefore alimit is set for the realization of integrated low-pass filters inmonolithic integration technology. After this limit, one must work witha large capacitor which is to be connected from the outside to theremaining integrated portion of the protection device for reasonsrelating to costs and layout.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an electric circuitcomprising a protection device for protection against interferencesignals, which protection device can be completely monolithicallyintegrated at a reasonable expense and which, nevertheless, performs ahigh filtering action against interference signals.

This object is met by the provision of a protecting device whichincludes a power source, an emitter follower connected to the powersource and including a transistor whose collector-emitter path isconnected in series with a first current source and whose base isconnected to a source of the input signals, a low-pass member includinga series connection of a second current source and a capacitor, theseries connection being connected to the power-source, and the junctionpoint of the capacitor with the second current source being connectedvia a diode to the first current source and to the input of aSchmitt-trigger circuit. The output of the Schmitt-trigger is connectedto an input of the electric circuit.

The electrical circuit according to the present invention has theparticular advantage that it requires a particularly low expenditure onstructural component elements. Accordingly, the integration isparticularly inexpensive, since the integrated protection devicerequires very little surface area. Moreover, the protection device,according to the invention, has a particularly high resistance totemperature changes. The monolithic integration construction provides avirtually ideal synchronizing for the emitter follower and the diode, sothat very few errors occur during the transmission of a constant-voltagelevel.

BRIEF DESCRIPTION OF THE DRAWING

The invention both as to its construction so to its method of operation,together with additional objects and advantages thereof, will be bestunderstood from the following description of the preferred embodimentswith reference to the accompanying drawings wherein:

FIG. 1 shows a basic circuit diagram of a protection device, accordingto the invention, which is series connected with an input of asubsequent electric circuit arrangement;

FIG. 2 shows a particularly simple embodiment of a protection deviceaccording to the invention which is suitable for a completely monolithicintegration;

FIG. 3 shows a modification of the capacitive means of FIG. 1 forincreasing the capacitor effect of an integrated barrier layercapacitor; and

FIG. 4 shows an embodiment of a protection device according to theinvention with a particularly high capacitor effect.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a basic circuit diagram of a protection device according tothe invention with a signal input 1, which is operated between a supplyvoltage line 2 and a ground line 3. The signal to be applied to theinput 1 leads to the base of a transistor 4 which is connected to act asan emitter follower. In addition, base a voltage divider consisting oftwo resistors 5, 6 is connected between the supply voltage line 2 andthe ground line 3. The emitter of the transistor 4 leads to the groundline 3 via an emitter current source 7. In addition, it is connectedwith the cathode of a diode 8 whose anode leads to the supply voltageline 2 via a charging current source 9. A capacitor 10 is connectedbetween the junction point a of the diode 8 and the charging currentsource 9, and the connection point b to the ground line 3. Further, thejunction point is connected with the input of a Schmitt trigger 11,whose output is connected with the input 12 of an electric circuitarrangement 13, which is not shown in more detail for the sake ofsimplicity.

In the inactive state of the protection device, i.e. when there is nosignal at the input 1, the potential at the base of the transistor 4 isdetermined by means of the divider ratio of the base voltage divider 5,6 and the potential of the supply voltage line. Since the voltage dropacross the base-emitter diode of the transistor 4 is compensated for bythe same drop in voltage across the diode 8, the potential at the inputof the Schmitt trigger 11 is substantially equal to the potential at thebase of the transistor 4. The hysteresis characteristic line of theSchmitt trigger 11 is dimensioned in such a way that in the inactivestate of the device there is an unequivocal potential at the input 12which is approximately equal to the potential at the ground line.

During a sudden positive pulse-shaped signal at the input 1, thepotential at the emitter of the transistor 4 connected as an emitterfollower follows the step-shaped signal. Since the potential at thecapacitor 10, and accordingly at the anode of the diode 8, has beendetermined by the previous potential, the diode 8 becomes blocked. Thetransistor 4 accordingly takes over the entire current of the emittercurrent source 7, while the entire current of the charging currentsource 9 is integrated by the capacitor 10. This process takes placeuntil the voltage at the capacitor 10 matches the potential at thesignal input 1.

On the other hand, during a sudden negative pulse-shaped signal at theinput 1, the potential at the emitter of the transistor 4 remainsintact, since it cannot drop below the voltage at the capacitor 10 whichis reduced by the flow voltage drop of the diode 8. The transistor 4forming the emitter follower is thus blocked. Current is then drawn offfrom the capacitor 10 by the current source 7 via the conductive diode 8until the potential at the input of the Schmitt trigger 11 equals thepotential at the signal input 1.

During a lower frequency rectangular signal at the input 1 of theprotection device according to FIG. 1, a trapezoidal signal thus occursat the input of the Schmitt trigger 11. The steepness of edges of thetrapezoidal signal at the input of the Schmitt trigger 11 is dependenton the currents supplied by the emitter current source 7 and thecharging current source 9 and on the magnitude of the capacitor 10. Thetrapezoidal voltage signal is exactly symmetrical when the currentintensity of the current flowing through the emitter current source 7 isexactly twice the intensity I of the current flowing through thecharging current source 9. The Schmitt trigger 11 responds to atrapezoidal input signal in a known manner in such a way that arectangular signal, which is delayed relative to the input signal and isused for controlling the circuit arrangement 13, appears at its output.

Thus, the entire protection device acts like a low-pass filter which isconnected prior to the input 12 of the circuit arrangement 13.Therefore, it offers a particularly good protection against pulseinterference signals of higher frequency as they frequently occur duringoperation in a motor vehicle. Therefore, the protection device,according to the invention, is applied chiefly in integrated controllingmeans for rotary current generators aboard a motor vehicle. However, theprotection device, according to the invention, is usable in a universalmanner and is suitable both for analog and digital signals.

A simplified form of the circuit arrangement according to the basiccircuit diagram shown in FIG. 1, which is suitable particularly for amonolithic integration, is shown in FIG. 2. For the sake of simplicity,no base voltage divider, no Schmitt trigger, and no circuit arrangement,is shown.

The protection device shown in FIG. 2 comprises a signal input 1 and isoperated between a supply voltage line 2 and a ground line 3. As in FIG.1, the input signal leads to the base of a transistor 4 which acts as anemitter follower. The diode 8 is realized by means of a transistor whosecollector and base are short circuited with one another in order to formthe anode of the diode 8. The capacitor 10 is now formed by a capacitor101, which is connected with the ground line 3, and by a capacitor 102which is connected with the supply voltage line 2. The total capacitanceis thus represented by the sum of capacitances of the capacitors 101,102.

The emitter current source 7 and the charging current source 9, of FIG.1, are now dependent on a single current source 70 in the protectiondevice shown in FIG. 2. The current source 70 is formed in the simplestembodiment form by a single ohmic resistor. It supplies current from thesupply voltage line 2 via a transistor 71 to the ground line 3, thetransistor 71 being connected as an input diode of a current mirrorarrangement. The current mirror arrangement comprises two outputtransistors 72, 73. The current flowing through the collector of theoutput transistor 72 serves to control the charging current source. Thecurrent flowing through the collector of the output transistor 73 is thecurrent of the emitter current source, for which purpose the collectorof the output transistor 73 is connected with the emitter of the emitterfollower 4. The emitter surfaces of the transistors 71, 72, 73 aredimensioned in such a way that the output current through the transistor73 is twice as great as the output current through the transistor 72.

The charging current source 9 of FIG. 1 is formed, in a known manner, bya current source with three transistors 91, 92, 93. As alreadymentioned, the control current of this current source is provided by theoutput transistor 72. The emitters of the transistors 91, 92 areconnected with the supply voltage line 2. The collector of thetransistor 91 is connected with the collector of the transistor 72 andthe base of the transistor 93. The emitter of the transistor 93 isconnected with the collector of the transistor 92 and the bases of thetransistors 91, 92. The collector of the transistor 93 again leads tothe anode of the diode 8.

The operation of the circuit shown in FIG. 2 corresponds to that of thebasic circuit shown in FIG. 1, and a repeated description is dispensedwith here for the sake of simplicity.

The capacitor 10 and the capacitors 101, 102, respectively, can berealized in desired variants in integrated technology, e.g. in MOStechnology or as barrier layer capacitors. When realized the capacitorsas barrier layer capacitors, the use of two capacitors, according to thearrangement according to FIG. 2, is particularly advantageous, since abarrier layer capacitors are dependent on the applied voltage. Thiseffect is compensated for by a parallel connection of the two capacitors101, 102. The compensation of the reciprocal blocking currents islikewise particularly advantageous. In addition, interference in thesupply voltage line 2 caused by the capacitive voltage division at theinput of the Schmitt trigger 11 shown in FIG. 1, is only effective to areduced extent.

An arrangement for increasing the capacitor action of a barrier layercapacitor is shown in FIG. 3. For this purpose, the barrier layercapacitor is formed as a capacitor diode 103 which is operated in theblocking voltage direction and is connected in series to theemitter-collector junction of a transistor acting as an input diode 104of a current mirror arrangement 104, 105. The current ratio of thecurrent mirror arrangement 104, 105 is adjusted by the emitter surfaces.If the ratio of the surface of the emitter of the input diode 104 to thesurface of the emitter of the transistor 105 is selected so as to equal1 /n, the two-terminal network shown in FIG. 3 appears as a barrierlayer capacitor 101 with a magnitude which is n+1 times that of thebarrier layer capacitor 103 because of the charging currentamplification. The terminals a', b' are connected to points a, b in FIG.1.

In order to discharge the barrier layer capacitor 103, a diode 106 isprovided which is connected to extend transverse to the base-emitterdiodes of the current mirror circuit 104, 105. Naturally, the barrierlayer capacitor 103 does not appear amplified by the factor n+1. Inmonolithic integration of current mirror-arrangement with n-p-ntransistors, the diode 106 can be formed by the collector-substratediode of the input diode 104.

These response thresholds of the capacitor amplification, which arecaused by the base-emitter voltages of the current mirror arrangement orof the diode, can lead to an undesirable falsification of the outputsignal when using the protection device for analog signals. Anembodiment of the invention which does not have these disadvantageousresponse thresholds of the capacitor amplification is shown in FIG. 4.

The protection device shown in FIG. 4 is provided with a signal input 1,as in the previous FIGS. 1 and 2, and is operated between a supplyvoltage line 2 and a ground line 3. The input signal is fed to anemitter follower 4, in a manner already described, whose emitter isconnected with the cathode of a diode 8. The anode of the diode 8 leadsto two barrier layer capacitors 101, 102 corresponding to the drawingshown in FIG. 2.

The barrier layer capacitor 101 leads from the anode of the diode 8 tothe ground line 3 via the collector-emitter junction of a transistor716. The base of the transistor 716 is connected with the base of atransistor 73 whose collector-emitter junction leads from the emitter ofthe emitter follower 4 to the ground line 3. The collector of thetransistor 716 is connected with the base of the transistor 73 via thebase-emitter diode of a transistor 715. The collector of the transistor715, on the other hand, is connected to the distribution voltage line 2.The transistors 716, 73 thus form a current mirror, arrangement whereinthe transistor 715 serves in a known manner as a base current amplifierin order to achieve an improved synchronization characteristic betweenthe transistors 716, 73. The barrier layer capacitor 101 is connected inthe input current path of the current mirror 716, 73, while the outputcurrent path is connected with the emitter follower 4 and the diode 8,respectively.

The barrier layer capacitor 102 is operated in a manner which iscompletely symmetrical to the latter, the depletion-layer capacitor 102being connected with the supply voltage line 2 via the collector-emitterjunction of a transistor 718. A transistor 717, whose base-emitter diodeis connected parallel to the collector-base junction of the transistor718 and whose collector leads to the ground line 3, serves as a currentamplifier. A current mirror arrangement 718, 94, whose output currentpath is connected with the anode of the diode 8, is formed by atransistor 94.

A comparison of the drawings shown in FIGS. 4 and 1 clearly shows thatthe transistor 94 in FIG. 4 corresponds to the charging current source 9in FIG. 1, while the transistor 73 in FIG. 4 corresponds to the emittercurrent source 7 in FIG. 1. In addition, it is stated above in thedescription of FIG. 1 that a particularly symmetrical operating behaviorof the protection device, according to the invention, is achieved inthat the intensity of the current supplied by the emitter current source7 is approximately twice as great as the current intensity of thecurrent supplied by the charging current source 9. This behavior is alsoadjusted in the protection device shown in FIG. 4 in that the transistor718 is acted upon with a constant current I, whereas the transistor 716is acted upon with an additional constant current 2I having twice theintensity of the first constant current I. These two constant currentsI, 2I are generated in a known manner by casoading a plurality ofcurrent mirrors arrangement from a single current source 700. Thecurrent source 700 can be realized in the simplest embodiment form by asingle ohmic resistor. For this purpose, the current of the currentsource 700 flows from the supply voltage line 2 to the ground line 3 viaa transistor which is connected as input diode 710 of a current mirrorarrangement. The current mirror arrangement comprises two outputtransistors 711, 712, whose emitters are connected with the ground line3, in each instance. The collector of the output transistor 712 isconnected with the base of the base current amplifier 717. The collectorof the output transistor 711 leads to a current mirror arrangement 713,714 which is operated by the distribution voltage line 2, wherein thetransistor 713 is operated as an input diode from the collector currentof the transistor 711. The collector of the output transistor 714 leadsto the base current amplifier 715. The emitter surfaces of the currentmirror arrangement 713, 714 and of the current mirror arrangement 710,711, 712 are adjusted in such a way that the output current 2I of thetransistor 714 is exactly twice as strong as the output current I of thetransistor 712.

In the drawing shown in FIG. 4, the capacitors appear amplified in eachinstance by the translation ratio of the current mirror arrangment 718,94 and 716, 73, respectively. Current flows continuously through thearrangement shown in FIG. 4. Changes in potential at the anode of thediode 8 lead directly to an additional triggering of the current mirrorarrangement via the charged barrier layer capacitors 101, 102. Sincethis occurs at a circuit intersection by current addition duringincrease in potential as well as during a drop in potential at the anodeof the diode 8, the arrangement in FIG. 4 does not have thedisadvantageous response thresholds for the capacitor amplificationdescribed with reference to FIG. 3.

We claim:
 1. A device for protecting input signals of an electriccircuit against interference pulses of a higher frequency, comprisingapower source having a first terminal and a second terminal; an emitterfollower including a transistor having its base connected to a source ofthe input signals, and a first series connection of thecollector-emitter path of said transistor with a first current source;said first series connection being connected between said first andsecond terminals; a low-pass member including a second series connectionof a second current source and a capacitor means; said second seriesconnection being coupled between said first and second terminals; acompensating diode having its anode connected to junction point of thesecond current source with the capacitor means and its cathode connectedto a junction point of the first current source with the emitter of thetransistor; and means for applying a signal present at said capacitormeans to an input of the electrical circuit.
 2. A device according toclaim 1 wherein said first and second current sources are formed ascurrent mirror circuits.
 3. A device according to claim 2 wherein saidcurrent mirror circuits comprise a first current mirror circuit having afirst input diode connected in series with a constant current source, afirst output transistor forming said first current source, and a firstcontrol transistor; said series connection of the constant currentsource and the first input diode being connected between said first andsecond power source terminals; a second current mirror circuit having aseries connection of a second input diode and a second outputtransistor, and a second control transistor, the series connection ofthe second input diode and the second output transistor being connectedbetween the first power source terminal and said compensating diode; acollector of said second control transistor being connected to the baseof said second output transistor and to the collector of said firstcontrol transistor; the base of said first control transistor beingconnected to a node between said constant current source and said firstinput diode; and emitters of said first- and second control transistorsbeing connected to said second first and first power source terminals,respectively.
 4. A device according to claim 3 wherein said firstcurrent source supplies a first constant current having a firstintensity value and said second current source supplies a secondconstant current having a second intensity value, the first intensityvalue being approximately two times higher than the second intensityvalue.
 5. A device according to claim 3, wherein said capacitor meanscomprises a first capacitor connected between said anode of thecompensating diode and one of said power source terminals, and whereinthe device further comprises a second capacitor connected between saidanode of the compensating diode and an other of the power sourceterminals.
 6. A device according to claim 1, said second seriesconnection further comprising means for amplifying charging anddischarging current through said capacitor means.
 7. A device accordingto claim 6 wherein said capacitor means is a capacitor diode; saidamplifying means including an input diode of a current mirror connectedin series with said capacitor diode; said current mirror including anamplifying transistor whose base is connected to the junction point ofsaid capacitor diode and said input diode and whose collector-emitterpath is connected parallel to said series connection of the capacitordiode and the input diode.
 8. A device as defined in claim 7 furthercomprising a discharging diode connected antiparallel to said inputdiode.
 9. A device according to claim 1 wherein said compensating diodecomprises a base-emitter diode of a transistor.
 10. A device accordingto claim 1, wherein the second current source is a resistor.
 11. Adevice according to claim 1 wherein the applying means comprises aSchmitt-trigger circuit having an input connected to the junction pointof the second current source and the capacitor means, and an outputconnected to the input of the electrical circuit.